The present invention relates to thermoplastic film structures, in particular plastic film structures which have been formed utilizing coextrusion techniques. The laminate comprises a core of a linear low-density polyethylene having exterior skin layers of low-density polyethylene, i.e., conventional polyethylene prepared utilizing the prior set free-radical high pressure polymerization process.
The use of thermoplastic stretch-wrap for the overwrap packaging of goods, in particular the unitizing of pallet loads, is a currently commercially developing end use application for thermoplastic films, including polyethylene. There are a variety of overwrapping techniques which are employed utilizing such stretch-wrap films, including locating the pallet load to be wrapped atop a rotating platform. As polyethylene film is laid on about the girth of the pallet load, the pallet load is rotated on its platform. The polyethylene stretch-wrap is applied from a continuous roll thereof. Braking tension is applied to the continuous roll of film so that the film is being continuously stretched by the rotating pallet load. Usually the stretch-wrap film located adjacent to the rotating pallet load is vertically positioned and the rotating platform or turntable may be operated at speeds ranging from about 5 up to about 50 revolutions per minute. At the completion of the overwrap operation the turntable is stopped completely while the film is cut and attached to the previous layer of film employing tack sealing, tape, spray adhesives or a cling-modified film whereby overlapping layers of the stretch-wrap have a pronounced tendency to cling together at their interface. Depending upon the width of the stretch film roll, the load being overwrapped may be shrouded in the film while the vertically positioned film roll remains fixed in a vertical position, or the vertically positioned film roll (e.g., in the case of relatively narrow film widths and relatively wider pallet loads) may be arranged to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Stretch films employed in the prior art have included film materials such as polyethylene, polyvinyl chloride and ethylene vinyl acetate.
With respect of the ethylene vinyl acetate type of stretch film products, the prior art has employed a percentage of weight of vinyl acetate in the co-polymers of about 2% up to about 15% and preferably from about 4% up to about 12% by weight for stretch film applications.
Physical properties which are particularly significant for the successful use of thermoplastic films in stretch-wrap applications include their puncture resistance, their elongation characteristics, their toughness, and their resistance to tearing while under tension. In particular, the latter physical characteristics of such film, i.e., their resistance to tearing and their resistance to puncture, are particularly significant. In general tensile toughness is measured as an area under a stress-strain curve for a thermoplastic film, or it may be considered as the tensile energy absorbed, expressed in units of ft.-lbs./in.cu. to elongate a film to break under tensile load. In turn, this toughness characteristic is a function of the capacity of such films to elongate. The process of stretching the film decreases that capacity. Accordingly, the stretch-wrap process will decrease the toughness of the film while it is in its stretched condition as an overwrap as compared to unstretched counterparts, including such materials as shrink-wrap. Generally this loss of toughness is proportional to the amount of stretch imparted to the film as it is overwrapping a load of goods.
As hereinabove indicated, the resistance to tear characteristic of such films will be obviously an important physical characteristic for stretch-wrap applications since if the edge of the stretch film roll is nicked, abraded or in any way weakened before stretching or during the stretching operation, premature tearing of the film will usually occur during wrapping or subsequent handling of the load of goods.
In practice, one commonly accepted technique for properly tensioning a film around a load such as a palletized load is to adjust the braking force on the roll until a significant amount of neck-in (i.e., film width reduction) occurs. Alternatively film may be tensioned until an initiated tear results in unrestricted propagation of the tear across the film width.
It is an object of the present invention to provide a stretch film material which, unlike currently commercially available stretch films, is a laminar structure comprising at least two and preferably three film layers. The prior set stretch film materials hereinabove referred to, such as polyvinyl chloride, ethylene vinyl acetate co-polymer and low-density polyethylene, have been found to offer reduced resistance to tear in both the film's machine direction and transverse direction as well as reduced toughness and elongation characteristics in contrast to the laminar film compositions of the present invention.